Surgical techniques for the treatment of spinal injuries or deformities are usually aimed at joining together two or more vertebrae of the spine, through a process that is called spinal fusion. A possible approach to spinal fusion adopts a fixation system that is anchored to the spine by way of orthopedic screws implanted into the pedicles of two or more subsequent vertebrae. The single screws are connected together by means of rigid or semi-rigid rods, which are conveniently housed within a transversal hole provided in the screw head.
However, due to the irregularity of bone anatomy, it is unlikely that once the screws have been implanted into the spine pedicles, their heads will be properly aligned for rod insertion. Hence, in order to facilitate the insertion of the rod, the screws are usually provided with a head that is freely rotatable with respect to their shank.
Screws of this type, named polyaxial screws, present a threaded shank with a bulging end that rotates in a socket-like cavity provided in the head. The upper part of the socket-like cavity is defined by a locking insert which is suitable to clamp the bulging end once the appropriate orientation of the shank has been set. The transversal hole for housing the connecting rod is arranged above the socket-like cavity, and a set-screw is provided above in order to clamp the rod into position. In typical polyaxial pedicle screws, such as the one disclosed in U.S. Pat. No. 5,672,176, the locking action of the set-screw determines the locking of both the connecting rod and the shank orientation, since the pressure applied by the set-screw is transmitted through the connecting rod to the locking insert.
However, such an arrangement may oblige the surgeon to insert the connecting rod while the screw head is freely rotatable with respect to the shank, which may be prejudicial to an optimal placement of the fixation system. As will be appreciated by those skilled in the art, spinal surgery, especially when adopting minimal invasive techniques, may be challenging and may be important to provide freedom of placement of the fixation systems together with ease of intervention.
An approach to the above-addressed issue is disclosed in U.S. Pat. No. 5,443,467, which discloses a peripherally threaded locking insert engaging with an internal threaded surface of the screw head. The shank orientation may thus be locked separately by the locking insert alone. However, such an approach does not allow the surgeon to correct the shank orientation during or after insertion of the connecting rod, since he would be unable to loosen the locking insert when the rod is placed upon it.
A different approach is disclosed in U.S. Pat. No. 7,223,268 and U.S. Pat. No. 6,063,090. These polyaxial screws have two concentric inner and outer upper set-screws: the outer one is intended to act on a portion of the locking insert, tightening it into position independently from the connecting rod, which is clamped by the inner one. However, the presence of the outer set-screw prevents insertion of the connecting rod from above, and as a consequence, the surgeon cannot lock the relative orientation of the shank before rod insertion in minimal invasive surgery.
Another approach is disclosed in U.S. Pat. No. 5,681,319, wherein a dedicated instrument is employed to keep the locking insert in clamping position during the setting of the connecting rod. However, this instrument has to be removed before insertion of the set-screw clamping the rod, once again limiting the surgeon's freedom in tightening and loosening the elements while looking for the best possible arrangement for the fixation system.
Another approach is disclosed in U.S. Pat. No. 6,063,089. This polyaxial screw has, a head having a side arm for the housing and separate locking of the connecting rod. However, such an arrangement may significantly increase the lateral profile of the fixation system, and may not suitable for traditional surgery techniques. Moreover, the position of the rod being off-set from the screw axis, the screw is also unsuitable for minimal invasive and deformity correction surgeries.